Hybrid electric vehicles are well known, and generally comprise an internal combustion engine, an electric motor, a battery and a control system for determining whether the vehicle is driven by the engine, the motor or a combination of engine and motor. The battery may be charged by a vehicle generator, or by the motor acting as a generator, for example during regenerative vehicle braking. In some circumstances the engine may drive the motor (as generator) to charge the battery.
Many factors influence how and when the vehicle battery should be charged when the vehicle is in use. For example account must be taken of the optimum state of charge of the battery, and the likely availability of opportunistic re-charging, for example by regenerative braking (at no fuel cost). The nominal state of charge may vary according to whether a vehicle driver has selected a program which is optimized for maximum fuel economy, maximum range or maximum performance. Charging of the battery may be controlled by a torque control system having a plurality of charging maps for each vehicle operating mode. In a hybrid electric vehicle, operating modes may include normal, electric (giving extended range under electric power), and sport (giving increased acceleration).
Charging maps may be determined empirically for a particular vehicle type or specification, and are not the subject of this invention as such.
Batteries used in hybrid vehicles have a relatively high voltage and certain characteristics, in particular they should in general not be fully discharged (0%) nor should they be repeatedly fully charged (100%), since this can be detrimental to battery life. The theoretical range of charge is less than 0-100%, and the useable range of charge may be smaller still, typically 25-75%.
In one embodiment the nominal state of charge of the battery may be at around 50%. This allows headroom for opportunistic recharging up the maximum usable state of charge of approximately 70%, and also allows the battery to operate at its most efficient state of charge.
The nominal state of charge may vary according to the operating mode of the vehicle, and the corresponding charging map.
The state of charge is generally indicated to the vehicle driver by means of a dashboard or instrument panel display, which may show a maximum and minimum corresponding to the range of charge which is permitted by the torque control system.
Typically the vehicle driver is not presented with an indicator of the full theoretical range of charge because to do so would give a misleading impression that the battery could not reach a desired ‘maximum’ charge. As noted above maximum theoretical charge is not desirable.
Accordingly the vehicle driver is generally presented with a display or indicator representing in effect a reduced range, in which the displayed minimum may correspond to 25% charge, and the displayed maximum to 75% charge.
Nevertheless, considering that a control allowance is included for opportunistic recharging, and to permit different levels of nominal charge, a conventional charge indicator may typically reach the maximum charge indication only seldom.
The present invention provides a method of displaying the state of charge and a display which addresses this circumstance, and gives an appropriate indication of the state of charge to the vehicle driver.